The present invention relates to a high power semiconductor module device and, more particularly, to a high power semiconductor module device constructed in such a manner that a circuit board to which semiconductor pellet are bonded is bonded onto a heat sink, and a tubular insulating case surrounding the sides of the circuit board is mounted on the heat sink; this device is used for instance for IGBT (insulated gate type bipolar transistor) pellets, diode pellets, etc.
FIG. 8A is a sectional view taken along the line VIIIA--VIIIA in FIG. 8B, schematically showing an example of the sectional structures of conventional IGBT module devices, and FIG. 8B is a schematic view showing a perspective plan view when the interior of the module device is viewed from a section taken along the line VIIIB--VIIIB in FIG. 8A.
Referring to FIGS. 8A and 8B, a circuit board 90 is constituted in such a manner that, on both the front and rear surfaces of an insulation substrate (such as a ceramic substrate composed of an aluminum nitride, alumina or the like) 91, metal plates, generally copper plates, 92 are attached. The copper plate 92 on the front surface side of the insulation substrate is patterned; that is, the copper plate 92 on the front surface side comprises at least a copper plate pattern 92a as a pellet mounting portion and a copper plate pattern 92b for connection of the external terminal. The reference numeral 93 denotes IGBT pellets bonded by soldering onto the copper plate pattern 92a as a pellet mounting portion on the front surface side of the insulation substrate 91; for instance in this conventional example, two pellets are provided as shown in FIG. 8B.
Numeral 94 denotes bonding wires (generally composed of aluminum) arranged in such a manner that the respective ends thereof are bonded to the electrode pads of the IGBT pellets 93 and the external terminal connecting copper plate pattern 92b on the front surface side of the insulation substrate 91 so as to electrically connect the electrodes pads and the copper plate patterns 92b on the front surface side of the insulation substrate 91 to each other.
Numeral 95 denotes a heat sink comprising a copper plate, to the upper surface of which the copper plate pattern 92 disposed on the rear surface side of the insulation substrate 91 is bonded by soldering.
Numeral 96 denotes a tubular insulation case (composed of for instance PBT, i.e., polybutylene telephthalate) mounted on the heat sink 95 in a state surrounding the sides of the circuit board 90. Numeral 97 denotes an insulating material (such as, e.g. gelatinous silicone rubber) disposed to fill the interior of the case 96, covering the IGBT pellets 93, the bonding wires 94 and the circuit board 90.
Numeral 98 denotes an insulating cover (composed of for instance PPS, i.e. polyphenylene sulfide) mounted on the upper surface of an opening of the case 96 to cover the upper surface of the opening. Numeral 99 denotes external connection terminals the upper end portions of which extend outwardly from the module device through the cover 98. The outwardly extended end portions of the external terminals each generally have an enlarged width and are each bent to extend along the upper surface of the cover 98. The lower end portion of each external terminal is bonded by, e.g brazing to the corresponding external terminal connecting copper plate pattern 92.
In the case of the structure of the above-described conventional module device, when the pellets are operated, heating and cooling are repeated, as a result of which the temperature of the whole module is repeatedly raised and lowered. In this case, the solder bonded portions of the circuit board 90 and the pellets 93, the solder bonded portion of the circuit board 90 and the heat sink 95, and the bonded portions of the bonding wires 94 are subjected to thermal fatigue, resulting in the destruction of the module device in some cases.
That is, the differences among the thermal expansion coefficient (4.times.10.sup.-6 /.degree. C.) of the silicon pellets for example, the thermal expansion coefficient (23.times..sup.10-6 /.degree. C.) of the bonding wires made of, e.g. aluminum, the thermal expansion coefficient (17.times.10.sup.6 /.degree. C.) of the copper (the copper patterns on the circuit board and the heat sink), the thermal expansion coefficient of the ceramic substrate (4.times.10.sup.-6 /.degree. C. in the case of an aluminum nitride ceramic substrate; 7.times.10.sup.-6 /.degree. C. in the case of an alumina ceramic substrate) result in causing differences among the amounts of expansion/contraction of the respective component portions, as a result of which the respective bonded portions tend to get loose or warped. In particular, when the temperature of the solder bonded portions rises, the solder is softened; and thus, the pellets and the circuit board tend to get afloat or loose.
In this connection, it should be noted that, through the tests of cyclically cooling and heating the module device over the temperature range of -40.degree. C. to +125.degree. C., it has been found that, in the case of the conventional module device, deteriorations such as the occurrence of cracks in the solder bonded portions are caused by around 300 cooling/heating cycles.
In the case of the conventional high power semiconductor module device, as mentioned above, when the temperature of the whole module is repeatedly raised and lowered due to the repetition of heating and cooling during the operation of the pellets, the solder bonded portions of the circuit board and the pellets, the solder bonded portion between the circuit board and the heat sink, and the bonded portions of the bonding wires are subjected to thermal fatigue, thus resulting in the destruction of the module device in some cases. This is quite a problem.